Electron gun and cathode



April 12, 1960 M, JEPPSON 2,932,755

ELECTRON GUN AND CATHODE Filed Jan. 50. 1956 what!!! FYLHME/VT SUP/ L VINVEN TOR. Mar/v5 IQ Jsppso/z BY 2,932,755 ELECTRON GUN AND CATHODEMorris R. Jeppson, Orinda, Calif., assignor to Applied RadiationCorporation, poration of California Walnut Creek, Califi, a cor- Thepresent invention relates to an improvement in cathode heatingparticularly adaptable to high power electronic tubes such as klystronsand the like.

Present day high power electron tubes such as klystrons for example areinherently simple and rugged and consequently have a very long lifeexcept for the electron emission portion thereof. Despite advances inthe cathode art it is common for the electron gun portion of klystronsto constitute the weak link wherein tube failure results from loss ofemission from the cathode surface or mechanical failure of the filamentheating the emission surface.

The present invention provides a simple and inexpensive solution to theabove-noted difficulty by divorcing at least a part of the cathode fromthe vacuum system of the tube. The invention provides a removablecathode cartridge that is easily and rapidly removed from orinserted ina high power tube whereby the portions of the tube that are mostsusceptible to damage or failure are readily replaceable withoutincurring appreciable periods of tube inactivity. There is additionallyprovided by the present invention a different and advantageous heatexchange medium whereby other and more desirable electron emissionsurfaces may be employed than had heretofore been feasible.

It is an object of the present invention to provide an improved electrongun structure for beam power tubes.

It is another object of the present invention to provide a replaceablecathode structure for electron tubes.

It is a further object of the present invention to provide conductionheating for indirectly heated electron emitters of electron tubes.

It is yet another object of the present invention to provide a cartridgecathode element adapted for insertion in and removal from high powerelectron tubes.

It is a still further object of the present invention to provide for anelectron tube improved cathode structure sealing olf a replaceablefilament element.

It is another object of the present invention to provide a replaceablecathode structure for continuously evacuated electron tubes and having asealed filament for heating the electron emitter by conduction.

It is still another object of the present invention to provide electrontube cathode structure including a removable gas filled filamentcartridge.

The present invention is herein disclosed in connection with particularpreferred embodiments thereof, however, the invention is not to belimited to specific details thereof and reference is made to theappended claims for a precise definition of the scope of the invention.

Preferred embodiments of the present invention are illustrated in theaccompanying drawing, wherein:

Figure 1 isa longitudinal section view of the electron gun structure ofa sealed off high power electron tube in accordance with the invention;and

Figure 2 is a longitudinal section view of the electron gun structure ofa continuously evacuated high power electron tube in accordance with thepresent invention.

United States Patent Considering now the structural details of anelectron tube, such as a klystron for example, there is provided as aportion thereof an electron gun 11 including an insulating high voltagebushing 12in the form of a hollow cylinder closed at the inner endthereof by a metal plate or anode 13 having a small transverse centralaperture 14. The other end of the cylindrical bushing is closed by ametal back plate or wall 16 having a large central opening transverselytherethrough providing communication with the interior of the bushing12. Formed integral with or attached to the back wall 16 is a largehollow metallic cylinder 17 disposed concentric with the bushing 12interior thereto and having the inner end thereof adjacent the anode 13.The cylinder 17 preferably has outwardly curved edges at the inner endthereof to prevent electron discharge between the anode and same and thewalls thereof may have a reduced thickness immediately behind the rolledinner edge, as shown, for reducing heat dissipation and conduction inthis area.

Of the above described elements which are generally conventional inklystrons, the anode 13 and cylinder 17 may be formed of stainless steeland the bushing 12 of suitable insulating material to withstand quitehigh voltages in vacuum. The bushing 12, anode 13, back wall 16 andcylinder 17 are joined together to form the shell of the electron gunand suitable provision (not shown) is made for mounting same in positionin a wall of the housing of a tube with which it is associated, the backwall 16 then forming a part of the outer wall of such housing.

Within the cylinder 17 adjacent the inner end thereof is disposed anelectron emitter 18 including a semispherical support member 19 attachedto the cylinder walls with the concave side thereof facing the anode 13.The support 19 is disposed in sealing relation to the cylinder walls asby welding thereto and an emitting material 21 is disposedover theconcave surface thereof. This structure then defines a cavity in thetube directed toward the anode from the tube exterior and sealed vacuumtight from the tube interior.

Indirect and replaceable heating means for the emitter 18 is provided inthe form of a cartridge 22 adapted to slide into the aforementionedcavity and to sealingly engage the back wall 16 of the tube for defininga heating chamber within the cylinder 17. This cartridge 22 includes athin walled cylinder 23 having a lesser diameter than the tube cylinder17 and being provided with an exterior flange 26 about the outer endthereof. The cartridge is attached to the tube with the cylinders 23 and17 in substantially coaxial alignment by bolts 27 through the cartridgeflange 26 threaded into bolt holes in the back wall 16 of the gunstructure. The cartridge is thus removably disposed with the cylinder 23thereof inside of the cylinder 17 and a suitable gasket 24 may beprovided about the flange 26 for sealing the interior of the cylinder 17at the outer end thereof. The outer end of the cartridge cylinder 23 isclosed by a disc 28 mounted in sealing relation to the cylinder. Thisdisc 28 supports in insulating relation a pair of filament leads 29 and31 extending therethrough via lead through insulators 32 and 33respectively. These leads 29 and 31 extend longitudinally of thecartridge cylinder 23 through a pair of insulating discs 34 and 36 toopposite ends of the valves then sealing an inert atmosphere within thecartridge. One or more openings may be provided transversely through theinsulating discs 34 and 36 for ready gas passage or the porosity of thediscs relied upon.

The filament 37 is supported in position behind the emitter 18 by meansof the filament leads 2-9 and 31 and these in turn are supported by theinsulating disc 36 contacting the filament wires. The other disc 34adjacent the filament has enlargerd transverse apertures through whichthe filament wires extend out of contact with the insulating disc.Inasmuch as the filament op- I crates in a gas atmosphere a lightweightceramic insulating material such as foamed ceramic may be used for thediscs 34 and 36. These insulating discs serve to support the filamentleads in insulated relation to the cartridge cylinder 23 and to provideheat insulation behind thefilament. The stepped apertures through theinsulating discs provide additional filament lead length betweenfilament and disc contact to allow maximum cooling of the leads beforecontact with the disc whereby disc damage is minimized. Of course thediscs 34 and 36 may be combined in a single unit with stepped transverseapertures therethrough.

Considering now the operation and use of the above described embodimentof the invention, the insulating bushing 12 together with attached anode13 and back plate 16 are mounted in a high power vacuum tube such as aklystron with suitable power supplies, not shown,

connected to maintain the anode 13 at a highly positive potential withrespect to the emitter 18 and surrounding cylinder 17, the latternormally being electrically grounded. A current supply 41 is connectedbetween the filament leads 29 and 31 causing a heating current to flowthrough the filament 37. The filament is heated by the current flowingtherethrough and emits heat by radiation to the support member 19 and byconduction through the atmosphere of inert gas also to the support 19.The gaseous atmosphere provides a good heat conducting medium betweenfilament 37 and support 19 so that the latter may be readily raisedto amuch higher temperature than is possible with radiation alone, as in avacuum. The small cross section of the cartridge cylinder 23 and outercylinder 17 limits heat transfer therealong toward the outer wall of thetube, here formed by the plate 1 6, and the insulating discs 34 and 36limit heat conduction back through the cartridge.

The elecron emission surface 21 is heated by the support 19 therefor soas to thermally emit electrons which are then attracted by therelatively positive potential of the anode 13 and these electrons arecollimated by the anode to pass in a beam through the aperture 14therein for additional operations within the remainder of the tube.Because of the improved heat transfer between filament and electronemitter it is possible to raise the temperature of the latter withoutforcing the filament to undesirable temperatures and it thus becomesfeasible to employ newer and more efiicient emission materials than hasheretofore been possible. Thoria emitters, for example, may be employedin tubes utilizing the present invention and more efiicient utilizationof cathode power is attained with any emission surface, such asconventional oxide emitters.

In operation of the tube the filament will most likely be the firstelement thereof to fail because of the particularly rugged constructionof tubes such as klystrons and despite the lower temperatures at whichit may be operated owing to the improved heat conducting atmospheresurrounding same. Conventionally, filament failure can only be repairedby complete tube disassembly which is a .16 of the electron gun 11.

major project requiring the tube to be removed from service for aconsiderable period of time. With the present invention, however, afilament may be replaced in the matter of a few minutes. The cartridge22 of the present invention is sealed-elf from the vacuum system of thetube so that it maybe removed without affecting the remainder of thetube. Removal of the bolts 27 frees the cartridge 22 so that it readilyslips out of the tube cylinder 17. With the cartridge removed thefilament 37 may be repaired or replaced to ready the cartridge forreinsertion in the tube or if time is of great importance a replacementcartridge may be used with the damaged one later repaired for subsequentuse as a spare part. The cartridge with a whole filament is theninserted in the tube cylinder 17 and aligned by inserting the bolts 27in the holes in the back plate 16. Tightening ofthe bolts fixes thecartridge and included filament in correct position. A source of inertgas 42 is then connected to one of the tubes 38 and with the valves inthe tubes 38 and 39 opened, gas is admitted to the cartridge interiorand same is purged of air so that an inert gas atmosphere remainstherein upon closure of the valves in the tubes 38 and 39. With theattachment of a filament supply 41 across the filament leads 29 and 31the tube is again in operating condition;

There are of course a multitude of variations. that may be made withinthe scope of the present invention and there is illustrated in Figure 2,as an example thereof, a unit suitable for use in a continuouslyevacuated tube as may be employed in connection with equipment havingevacuation means. In the embodiment of Figure 2 elements which are thesame as in Figure l are similarly numbered and a cartridge 22' isattached by bolts 27 through the cartridge flange 26 into holes in aback wall As is conventional, a high voltage bushing 12 surrounds theelectron gun and spaces the anode 13 from the back wall which has acylindrical portion 17 extending toward the anode with the inner rolledend thereof spaced therefrom.

The cartridge 22 includes a thin walled cylinder 51 similar to thecylinder 23 of Figure 1 but carrying at its inner end a flange 52 havingan outer diameter substantially equal to the inner diameter of the tubecylinder 17 for sliding therein. This flange 52 positions the cartridge22' coaxially within the cylinder 17 while providing for longitudinalmotion of the cartridge therein. The cartridge '22'further differs fromthe one previously described in that it includes the electron emitter18. The emitter support 19 is mounted in the inner end of the cartridgecylinder 51 with the emission surface 21' on the outer side of thesupport. By this means the electron emitter is made a part of theremovable cartridge 22' and the support 19 seals the inner end of thecylinder 51 to form a closed chamber.

The interior of the cartridge 22' is much the same as that previouslydescribed and there is included a filament 37 adjacent the emitter 18and separated from a back disc 28 by a pair of insulating discs 34 and36. Filament leads 29 and 31 maintained in position by the insulatingdiscs support the filament and extend to the back disc 28 where at leastone lead 29 is brought out of the cartridge via a lead-through insulator32 in the back disc 28. As a ground return may be employed forthefilament circuit the other filament lead 31 may be electricallyconnected to the back disc 28 which is grounded and a. filament supply(not shown) is connected between lead and ground. 7

It is contemplated in this modification ofthe invention that an inertgas atmosphere shall be provided between filament and emitter, and asealed system therefor is herein provided. The cartridge is adapted forfilliiig'with inert gas during assembly, or reassembly following repair,so that no gas need be added when the cartridge is inserted in the tube.The contained atmosphere of the cartridge may be provided through tubesof apertures sealed following assembly or final assembly may be made inthe desired atmosphere.

The above described embodiment of the present invention is employed as areplaceable unit of the continuously evacuated tube in which it ismounted. Upon filament failure or emitter poisoning as by exposure toair at a high temperature as may result from air leaks in the tube orassociated equipment, the cartridge is readily removed by threading outthe bolts 27. The cartridge including the emitter 18 at the inner endthereof is then slid out of the tube cylinder 17 and another new orrepaired cartridge slid in and secured. As the tube is continuouslyevacuated no harm results from opening the electron gun portion thereofto air for upon reinsertion of a cathode cartridge the gun is againsealed and continued pumping rapidly restores vacuum within the tube.This cartridge has the advantage of being a complete sealed unitcontaining a gaseous atmosphere so that no gas equipment is requiredduring cartridge replacement and it will be appreciated that such a unitmay be employed with sealed-cit tubes as by forming an inner cartridgeend to mate with the emitter support in heat exchange relationship, theillustrated embodiments being herein described merely as examples ofstructure in accordance with the invention.

What'is claimed is:

l. Cathode structure for an electron tube comprising an electron emitteradapted for indirect heating, wall structure defining together with saidemitter a sealed tube boundary, and an externally accessible cartridgeincluding a heating filament and heat shields behind same withenveloping wall structure open at one end adapted for removable sealingrelation to said tube adjacent the emitter thereof with said filamentadapted for electrical energization to heat said emitter whereby saidfilament is removable without afiecting tube vacuum.

2. Cathode structure as defined in claim 1 further characterized by saidcartridge having gas inlet and outlet means for admitting an atmosphereof inert gas and sealing same therein whereby heat from said filamentconducts through said gas to said emitter.

3. Cathode stnucture for an electronic vacuum tube comprising wallstructure defining a chamber, an externally accessible cartridge adaptedfor removable disposition in said chamber and including a heatingfilament adapted for electrical energization, and an inert gasatmosphere within said cartridge about said filament and sealed fromsaid tube by an electron emitter whereby the latter is heated by gasconduction from said filament.

4. An electron gun for a vacuum tube comprising wall structure defininga cavity with an electron emitter forming a part of said wall structureand sealing the tube interior from said cavity, a filament cartridgeopen at one end and adapted for externally removable disposition in saidcavity in sealing relation thereto to form a sealed chamber therein,said cartridge having a heating filament adapted for electricalenergization to emit heat therefrom, and an atmosphere of inert gasdisposed Within said chamber about said filament and extendingsubstantially to said emitter for heating the latter through conductionfrom the filament.

5. An electron gun for a vacuum tube comprising cylindrical wallstructure extending into the tube with the inner end thereof sealed byan electron emitter adapted for indirect heating and together defining acavity sealed from the tube interior and a cylindrical cartridge havingan externally accessible flanged outer end and open inner end adaptedfor removable insertion in said cavity with said flange sealing same,said cartridge having a heating filament at the inner end thereof andgas connections for filling with a gaseous atmosphere whereby saidfilament is externally removable from said tube without affecting tubevacuum.

6. An electron gun for a continuously evacuated electron tube comprisingan apertured anode; a high voltage bushing extending from said anode; anopen ended tube cylinder extending from the free end of said bushingtoward said anode at the aperture thereof and defining a cylindricalcavity into said tube from the exterior thereof; and a cathode cartridgeincluding a cylinder with an electron emitter sealing the inner endthereof adapted for externally removable insertion in said tube cavityin sealing relation to said tube cylinder, a filament within saidcartridge adjacent the emitter thereof, and .an atmosphere of inert ascontained within said cartridge between filament and emitter.

7. A filament cartridge for use with a vacuum tube comprising a thinwalled cylinder having an exterior flange about one end thereof, andopen at the other, a heating filament mounted in insulated relation tosaid cylinder adjacent the unfianged open end thereof, externallyaccessible attachment means for removably securing said cartridge to avacuum tube adjacent an electron emitter thereof, and sealing means forestablishing a vacuum seal between said fiange and vacuum tube tothereby define a chamber about said filament whereby same is adapted tocontain a gaseous atmosphere for conductive heat transfor to theelectron emitter.

8. A filament cartridge as defined in claim 7 further characterized byat least one insulating disc transversely of said thin walled cylinderwith said disc being heat insulating and electrically insulating, a pairof metal supporting leads extending through said disc in spaced relationand supported thereby with said leads joined to opposite ends of saidfilament and fixing the position thereof, and a cover sealing theflanged end of said thin walled cylinder with said leads extendingtherethrough in insulated relation thereto for attachment to a currentsupply to energize said fiiament.

9. A cathode cartridge comprising wall structure defining a gas tightchamber with a portion of the wall being electron emissive on the outersurface thereof, a heating filament mounted in insulated relation withsaid chamber adjacent the electron emissive wall thereof and adapted forelectrical energization to heat same, a gaseous atmosphere containedwithin said chamber about said filament for conducting heat to theelectron emissive portion of said wall structure, and externallyaccessible means removably securing the above-described parts of thecathode cartridge in a wall of a vacuum tube with the electron emissivesurface defining a portion of the wall of the vacuum tube for comprisingthe cathode thereof.

References Cited in the file of this patent UNITED STATES PATENTS2,410,822 Kenyon Nov. 12, 1946 2,432,789 OLarte et al Dec. 16, 19472,436,265 Pohle et al Feb. 17, 1948 2,471,298 Atlee May 24, 19492,545,884 Isaacs et al Mar. 20, 1951 2,611,883 Noel Sept. 23, 19522,625,670 Embshofif Jan. 13, 1953 2,666,160 Bowie Jan. 12, 19542,811,667 Brewer Oct. 29, 1957 FOREIGN PATENTS 121,599 Australia June18, 1946 658,747 Great Britain Oct.,10, 1951

